1. Field of the Invention
The present invention relates to an electric cable, in particular for power transmission or distribution at medium or high voltage, provided with a water swellable layer suitable to form a barrier to the water penetration in a position radially external to the metallic screen.
In the present description, the term “medium voltage” means a voltage of between about 1 kV and about 30 kV, while the term “high voltage” means voltages above 30 kV.
2. Description of the Related Art
Cables for power transmission or distribution at medium or high voltage generally have a metallic conductor coated, respectively, with a first inner semiconducting layer, an insulating layer and an outer semiconducting layer. In the course of the present description this predetermined sequence of elements will be indicated by the term “core”.
In a position radially external to the aforementioned core, said cable is provided with a metallic screen, usually of aluminium, lead or copper, inside which the aforementioned core is enclosed, said metallic screen generally consisting of a continuous tube or of a metallic tape shaped according to a tubular form and welded or sealed to ensure hermeticity.
Thus, said screen has a dual role: on the one hand it provides hermeticity against the exterior of the cable by interposing a barrier to water penetration in the radial direction, and on the other hand it performs a function of an electrical nature by creating, inside the cable, as a result of direct contact between the metallic screen and the outer semiconducting layer of said core, a uniform electric field of the radial type, and at the same time cancelling the external electric field of said cable. Yet another function is that of withstanding short-circuit currents.
In a configuration of the unipolar type, said cable has, finally, an outer polymeric sheath in a position radially external to the metallic screen mentioned above.
Cables for power transmission or distribution are, moreover, generally provided with one or more layers specifically placed to achieve a barrier effect able to block any water penetration towards the interior (the core) of the cable.
Ingress of water to the interior of a cable is a particularly undesirable aspect since, in the absence of suitable solutions designed to plug the water, once the latter has penetrated it is able to flow freely inside said cable. This is particularly harmful in terms of the integrity of the latter as problems of corrosion may develop within it, as well as problems of accelerated ageing of the cable itself with deterioration of the electric features of the insulating layer (especially when the latter is made of cross-linked polyethylene), a phenomenon that is better known by the term “water treeing” and that is manifested by the formation of microscopic channels in a branch structure (“trees”) due to the combined action of the electric field generated by the passage of current in the conductor, and of moisture that has penetrated inside said insulating layer.
This means, therefore, that in the case of water penetration to the interior of a cable, the latter will have to be replaced. Moreover, once water has reached joints, terminals or any other equipment connected to one end of the cable, the water not only stops the latter from performing its function, but also damages said equipment, in most cases causing damage that is irreversible and significant in economic terms.
Water penetration to the interior of a cable may occur through multiple causes, especially when said cable forms part of an underground installation. Such penetration can occur, for example, by simple diffusion of water through the outer polymeric sheath of the cable or as a result of abrasion, accidental impact or the action of rodents, factors that can lead to an incision or even to rupture of the outer sheath of the cable and, therefore, to the creation of a preferred route for ingress of water to the interior of the cable.
Numerous solutions are already known for tackling these problems. Of these, we may mention, for example, the use of hydrophobic and water swellable compounds, in the form of powders or gel, which are placed inside the cable at various positions depending on the type of cable being considered.
For example, said compounds may be placed in a position radially internal to the metallic screen, more precisely in a position between the cable core and its metallic screen, or in a position radially external to said metallic screen, generally in a position directly beneath the outer polymeric sheath, or in both the aforesaid positions simultaneously.
The water swellable compounds, as a result of contact with water, have the capacity to expand in volume and thus prevent longitudinal propagation of the water by interposing a physical barrier to its free flow. This solution thus makes it possible to restrict the damage to a section of cable of limited length, so that, once identified, the damaged section can be replaced safeguarding any equipment connected to it.
According to a known solution, for the purpose of facilitating the application of said water swellable material, the cable is provided with a tape, with which the aforementioned material is combined. In more detail, said tape is helicoidally or longitudinally wound on a suitable element of the cable itself, differing from one instance to another and depending, as mentioned, on the type of cable under consideration and on the positioning, radially internal or external to the metallic screen, of the barrier that is to be produced.
This tape can, for example, consist of a pair of fabrics of a cellulosic material, superimposed on one another in such a way that, in the space between said pair of fabrics, the water swellable powder as mentioned above can easily be placed; According to a different embodiment, for the purpose of reducing its thickness, said tape is made of a material that is sufficiently compressible and porous to permit trapping of a water swellable or super-absorbent material (see, for example, patent U.S. Pat. No. 4,867,526). According to a further embodiment of the known state of the art, said tape consists of a super-absorbent fibre material arranged linearly along the cable or helicoidally wound on said cable or on predetermined portions of the cable.
For the purpose of providing a barrier to water penetration in a position radially internal to the metallic screen, another known technique is to provide the outer semiconducting layer of the cable core with a plurality of longitudinal channels, preferably with V profile, in which a water swellable powder material is placed.
Document WO 99/33070 describes the use of a layer of expanded polymeric material arranged in direct contact with the core of a cable, in a position directly beneath the metallic screen of the cable, and possessing predefined semiconducting properties with the aim of guaranteeing the necessary electrical continuity between the conducting element and the metallic screen.
The technical problem faced in WO 99/33070 was that the covering layers of a cable are continuously subjected to mechanical expansions and contractions due to the numerous thermal cycles that the cable undergoes during its normal use. Said thermal cycles, caused by the diurnal variations in strength of the electric current being carried, which are associated with corresponding temperature variations inside the cable itself, lead to the development of radial stresses inside the cable which affect each of said layers and, therefore, also its metallic screen. This means, therefore, that the latter can undergo relevant mechanical deformations, with formation of empty spaces between the screen and the outer semiconducting layer and possible generation of non-uniformity in the electric field, or even resulting, with passage of time, in rupture of the screen itself.
This problem was solved by inserting, under the metallic screen, a layer of expanded polymeric material capable of absorbing, elastically and uniformly along the cable, the aforementioned radial forces of expansion/contraction so as to prevent possible damage to the metallic screen.
Furthermore, document WO 99/33070 discloses that, inside said expanded polymeric material, positioned beneath the metallic screen, a water swellable powder material is embedded, which is able to block moisture and/or small amounts of water that might penetrate to the interior of the cable even under said metallic screen.
Document WO 98/52197 describes the structure of a cable for power transmission comprising, preferably in a position directly beneath the outer polymeric covering sheath, a covering of expanded polymeric material of suitable thickness, capable of endowing the cable with high impact strength. This strength therefore makes it possible to eliminate the traditional metallic protective armours. Document WO 98/52197 makes no mention of the problem of water penetration to the interior of such a cable.